Medical imaging devices provide non-invasive methods to visualize the internal structure of a patient. Such non-invasive visualization methods can be helpful in treating patients for various ailments. For example, the early detection of cancer in a patient can be important in treating that patient. For most cancers, when detected at an early stage, the survival probability of the patient can increase.
In the U.S. breast cancer mortality is second only to that of lung cancer. Because of its role in early tumor detection, mammography has become the most commonly used tool for breast cancer screening, diagnosis and evaluation in the United States. A mammogram is an x-ray image of inner breast tissue that is used to visualize normal and abnormal structures within the breasts. Mammograms provide early cancer detection because they can often show a breast lumps and/or calcifications before they are manually palpable. One drawback of mammography is that it provides only a two-dimensional representation of a compressed breast, and as a result masses which are hidden by intervening structures may not always be readily discernible on a mammogram.
Tomosynthesis systems, which are x-ray systems for obtaining a three dimensional image volume of a breast, have recently been developed for use in breast cancer screening. One such tomosynthesis system, the Selenia® Dimensions® breast tomosynthesis system, is provided by Hologic, Inc., of Bedford Mass., the assignee of the present invention. In contrast to typical mammography systems, the tomosynthesis system acquires a series of x-ray projection images, each projection image obtained at a different angular displacement as the x-ray source traverses along a path over the breast. Reconstructed tomosynthesis slices reduce or eliminate the problems caused by tissue overlap and structure noise in single slice two-dimensional mammography imaging. Digital breast tomosynthesis also offers the possibility of reduced breast compression, improved diagnostic and screening accuracy, fewer recalls, and 3D lesion localization. Examples of breast tomosynthesis systems are described in U.S. Pat. Nos. 7,245,694 and 7,123,684, commonly owned by the Assignee of this application and incorporated by reference herein.
While mammography (and now tomosynthesis systems) have become the ‘gold standard’ for breast cancer screening, if the screening identifies a lump or a mass the standard protocol recommends review of the patient using a different imaging modality such as ultrasound imaging, to further characterize the mass or region of interest during breast cancer diagnosis.
Ultrasound imaging, another non-invasive medical imaging technique, uses sound waves, typically produced by piezoelectric transducers to image a tissue in a patient. The ultrasound probe focuses the sound waves, typically producing an arc-shaped sound wave which travels into the body and is partially reflected from the layers between different tissues in the patient. The reflected sound wave is detected by the transducer and converted into electrical signals that can be processed by the ultrasound scanner to form an ultrasound image of the tissue.
The typical procedure followed to obtain ultrasound images of a patient's breast involves positioning a patient in a supine position upon a table, applying a gel or other acoustic couplant to the patient's breast, and passing an ultrasound transducer across the patient's breast. As the transducer traverses the breast, ultrasound images can typically be viewed in real-time on a display of an ultrasound system. The ultrasound transducer may be either a hand-held transducer which is manually manipulated by the imaging technician, or may be an automated scanning device, such as that described in U.S. Pat. No. 7,731,662. One drawback of such methods lies in the fact that the breast is a very malleable structure; the geometry and structures of the breast move and change whenever the patient changes position. Thus, a mass which is readily identified when a patient is positioned for imaging using a first modality may be difficult to detect when the patient is repositioned for examination using a second modality.